$ B = \left[\begin{array}{rr}2 & 0 \\ -2 & 4 \\ -1 & 2\end{array}\right]$ $ D = \left[\begin{array}{rr}0 & -1 \\ 4 & 2 \\ 4 & 3\end{array}\right]$ Is $ B+ D$ defined?
Answer: In order for addition of two matrices to be defined, the matrices must have the same dimensions. If $ B$ is of dimension $( m \times  n)$ and $ D$ is of dimension $( p \times  q)$ , then for their sum to be defined: 1. $ m$ (number of rows in $ B$ ) must equal $ p$ (number of rows in $ D$ ) and 2. $ n$ (number of columns in $ B$ ) must equal $ q$ (number of columns in $ D$ Do $ B$ and $ D$ have the same number of rows? Yes Yes No Yes Do $ B$ and $ D$ have the same number of columns? Yes Yes No Yes Since $ B$ has the same dimensions $(3\times2)$ as $ D$ $(3\times2)$, $ B+ D$ is defined.